1. Field of the Invention
The present invention relates to a method for measuring the thickness of a film, and more particularly to a method for in-process sequential measurement of the thickness of a continuous film of a high molecular weight polymer during the production of the film of high molecular weight polymer by utilization of its optical characteristics. The present invention also relates to an apparatus containing an optical and electric arrangement to carry out the method.
2. Description of the Prior Art
Recently, high molecular weight polymer films, such as polyester film, have been finding use in an increasing number of fields. At the same time, the quality requirements for these films have become more severe. One of such requirements is for even thickness. Accordingly, in the production of continuous films, it is necessary to carry out in-process thickness measurement. A typical method for in-line measurement of film thickness, disclosed in Japanese Unexamined Patent Publication No. 53-31155, employs an infrared ray thickness measuring device which includes two alternately switchable optical filters, one being used for transmitting an infrared ray of a peak-absorption wavelength that is strongly absorbed by the film to be measured and the other being used for transmitting an infrared ray of a non-absorption wavelength that is substantially not absorbed by the film. The two infrared rays are alternately passed through the same optical path.
This known method is very effective for stable sequential thickness measurement of films, but suffers from a problem. Specifically, when a large change in the film thickness occurs or when the atmospheric temperature changes, the transmittance of the infrared rays of both the peak-absorption and non-absorption wavelengths does not changes accordingly, thereby causing errors in measurement of the film thickness. Also, with small thicknesses of the measured film, the amount of infrared rays absorbed by the film is small, resulting in low sensitivity and accuracy of thickness measurement and making it difficult to the detect changes in the thickness of the continuous film. Further, the alternate use of the infrared rays of the peak-absorption wavelength and the non-absorption wavelength makes it impossible to continuously measure the film thickness and limit the response speed in the measurement. The latter problems are particularly serious since the recent production speed of the films is as high as several hundred meters per minute and the response speed of the in-process measurement of films is more than several dozen milliseconds.